This invention relates to a manufacturing method for thin-plate magnets ideal for magnetic circuits used in various small motors, actuators, and magnetic sensors, etc. In the present invention, magnets are obtained, by the method of continuously casting a melt having a specific composition containing 6 at % or less of a rare earth element and 15 to 30 at % boron, on a turning cooling roller or rollers, in a prescribed reduced-pressure inactive gas atmosphere, so that the magnets have a crystalline structure wherein, in the state wherein they are cast, 90% or more is actually constituted by an Fe3B compound and xcex1-Fe coexisting with a compound phase having an Nd2Fe14B crystalline structure, and exhibit a fine crystalline structure having for each configuring phase a mean crystal grain diameter of from 10 to 50 nm. The present invention relates to a method of manufacturing thin-plate magnets, immediately from the alloy melt, having a fine crystalline structure with a thickness of 70 to 500 xcexcm and exhibiting magnetic properties of iHcxe2x89xa72.5 kOe and Brxe2x89xa79 kG.
Even higher performance, smaller size, and lighter weight are now being demanded in home electric appliances, automated office equipment, and electrical fixtures. In this context, design work is being done to maximize the performance-to-weight ratio in the entire magnetic circuits which use permanent magnets. In particular, in the structures of brush-type DC motors that make up the majority of the motors now being produced, permanent magnets having a residual flux density Br of 5 to 7 kG or so are considered ideal, but such cannot be obtained with conventional hard ferrite magnets.
Such magnetic characteristics can be realized in Ndxe2x80x94Fexe2x80x94B sintered magnets and Ndxe2x80x94Fexe2x80x94B bonded magnets wherein the main phase is Nd2Fe14B, for example. However, as they contain 10 to 15 at % of Nd which requires a great number of process steps and large-scale equipment for the metal separation refinement and reduction reactions, their cost becomes much higher than the cost of hard ferrite magnets, and, in view of the performance-to-cost ratio, they are only replacing hard ferrite magnets in some models. Currently, moreover, no inexpensive permanent magnet material which exhibit a Br of 5 kG or higher has yet been found.
Furthermore, in order to achieve smaller and thinner magnetic circuits, thin-plate permanent magnets wherein the thickness of the permanent magnet itself is on the order of 100 to 500 xcexcm are being sought. However, because of the great difficulty of obtaining bulk material for Ndxe2x80x94Fexe2x80x94B sintered magnets having a thickness of less than 500 xcexcm, such can only be fabricated by grinding a plate-form sintered body several mm in thickness or by the method of slicing the bulk material with a wire saw, leading to the problems of high cost and low yield.
Ndxe2x80x94Fexe2x80x94B bonded magnets are obtained by using a resin to bond together powder having a diameter of 10 to 500 xcexcm, wherefore it is very difficult to form bonded magnets having a thin-plate thickness of 100 to 300 xcexcm.
Recently, in the field of Ndxe2x80x94Fexe2x80x94B magnets, a magnet material has been proposed wherein an Fe3B compound is made the main phase with an Nd4Fe77B19 (at %) neighboring composition (R. Coehoorn et al., J. de Phys, C8, 1988, pages 669, 670). The details of this technology are disclosed in U.S. Pat. No. 4,935,074.
Even earlier, in U.S. Pat. No. 4,402,770, Koon proposed a method for manufacturing a permanent magnet made up of fine crystals wherein an Laxe2x80x94Rxe2x80x94Bxe2x80x94Fe amorphous alloy containing La as a mandatory element is subjected to a crystallizing heat treatment.
More recently, Richter et al. have reported producing amorphous flakes by spraying an Ndxe2x80x94Fexe2x80x94Bxe2x80x94Vxe2x80x94Si alloy melt containing 3.8 to 3.9 at % of Nd onto a turning copper roller, heat treating these flakes at a temperature of 700xc2x0 C., and thus obtaining thin pieces having hard magnetic properties, as disclosed in EP Patent Application 558691B1. These permanent magnet materials obtained by subjecting amorphous flakes having a thickness of 20-60 xcexcm to a crystallizing heat treatment have a metastable structure containing a crystalline aggregate structure wherein a soft magnetic Fe3B phase and a hard magnetic R2Fe14B phase are mixed.
The permanent magnet materials noted above exhibit a Br of around 10 kG and an iHc of 2 to 3 kOe, wherein expensive Nd is contained in a low concentration of 4 at % or so, wherefore the raw materials mixed in are less expensive than for Ndxe2x80x94Fexe2x80x94B magnets wherein the main phase is Nd2Fe14B.
In the permanent magnet materials described above, however, the rapid solidification conditions are limited because making the raw materials so mixed in to an amorphous alloy is a mandatory condition, and, at the same time, the heat treatment requirement for obtaining a hard magnetic material are narrowly limited. Hence these are not practical in terms of industrial production, and thus cannot be provided as inexpensive replacement products for hard ferrite magnets. Furthermore, such permanent magnet materials are obtained by subjecting amorphous flakes having a thickness of 20 to 60 xcexcm to a crystallizing heat treatment, wherefore it is not possible to obtain permanent magnets having a thickness of 70-500 xcexcm required for thin-plate magnets.
Meanwhile, U.S. Pat. No. 4,802,931 discloses a rapidly solidified Ndxe2x80x94Fexe2x80x94B magnet material consisting of a structure formed of a crystalline substance exhibiting hard magnetism that is directly obtained by rapidly solidifying an alloy melt on a roller rotating with a circumferential speed of about 20 m/s. However, the rapidly solidified alloy flakes obtained under these conditions have a thickness of around 30 xcexcm, wherefore, although they can be ground to a powder having a grain diameter of between 10 and 500 xcexcm or so and thus used in the bonded magnets described earlier, they cannot be used in thin-plate magnets.
An object of the present invention is to resolve the problems noted above in Ndxe2x80x94Fexe2x80x94B magnets containing 6 at % or less of a rare earth element and exhibiting fine crystallization. Another object thereof is to obtain, by casting, magnets exhibiting a performance-to-cost ratio comparable to hard ferrite magnets and exhibiting an intrinsic coercive force iHc of 2.5 kOe or greater and a residual magnetic flux density Br of 9 kG or greater. Yet another object thereof is to provide a method for manufacturing thin-plate magnets having a fine crystalline structure and thickness of 70 to 500 xcexcm wherewith it is possible to make magnetic circuits smaller and thinner.
The inventors previously disclosed (in Japanese Patent Application No. H8-355015/1996) how to obtain a fine crystalline permanent magnet exhibiting hard magnetic properties (iHcxe2x89xa72 kOe and Brxe2x89xa710 kG) directly from alloy melts, by a manufacturing method wherein alloy melts of a low-rare-earth Ndxe2x80x94Fexe2x80x94B ternary structure containing 6 at % or less of Nd and 15 at % to 30 at % of boron are continuously cast on a cooling roller turning with a roller circumferential speed of 2 to 10 m/s in a specific reduced-pressure inert or inactive gas atmosphere. There is a problem in this method of manufacturing Ndxe2x80x94Fexe2x80x94B ternary magnets, however, in that the roller circumference speed range must be narrowly limited to obtain the hard magnetism. In addition, in these Ndxe2x80x94Fexe2x80x94B ternary magnets, the best coercive force obtainable is on the order of 2 to 3 kOe. As a consequence, not only is the thermal demagnetization great, but it is necessary also to raise the operating point of the magnets as high as possible, whereupon problems arise due to limitations in terms of magnet shape and utilization environment.
The inventors thereupon conducted multifaceted research on the problems involved in manufacturing Ndxe2x80x94Fexe2x80x94B fine crystalline permanent magnets containing low rare earths wherein soft magnetic phases and hard magnetic phases are present together in a nano-mater size scale. As a result of this research, the inventors found that the problems noted above can be resolved by employing an alloy melt, to which a specific element has been added, in the process previously proposed by the inventors for manufacturing fine crystalline permanent magnets exhibiting a fine crystalline structure of 10 to 50 nm, directly from the alloy melt, by continuously casting the alloy melt on a turning cooling roller in a specific reduced-pressure inert or inactive gas atmosphere. With this method in which the alloy melt to which the specific element has been added is used, the iHc of the magnets can be improved to 2.5 kOe or better, the optimum roller circumferential speed range wherewith hard magnetic properties are obtained can be expanded, in comparison to manufacturing conditions conventional Ndxe2x80x94Fexe2x80x94B ternary magnet, and, at the same time, fine crystalline permanent magnets can be obtained having a thickness of 70 to 500 xcexcm. Thus was the present invention perfected.
More specifically, in the manufacturing method for thin-plate magnets having a fine crystalline structure according to the present invention, an alloy melt is used that is expressed by a composition formula of either Fe100xe2x88x92xxe2x88x92yxe2x88x92zRxAyMz or (Fe1xe2x88x92mCom)100xe2x88x92xxe2x88x92yxe2x88x92zRxAyMz (where R is one or more elements in the group Pr, Nd, Tb, and Dy, A is either C, or B, or C and B, and M is one or more elements in the group Al, Si, Ti, V, Cr, Mn, Ni, Cu, Ga, Zr, Nb, Ag, Hf, Ta, W, Pt, Au, and Pb), and that satisfies the conditions 1 xe2x89xa6x less than 6 at %, 15 less than yxe2x89xa630 at %, 0.01xe2x89xa6zxe2x89xa67 at %, and 0.001xe2x89xa6mxe2x89xa60.5 for the symbols x, y, z, and m delimiting the composition range, respectively.
In the present invention, this alloy melt is continuously cast on a cooling roller or rollers that rotate with a roller circumferential speed of 1 to 10 m/s (at an average cooling speed of from 3xc3x97103 to 1xc3x97105xc2x0 C./sec), in a reduced-pressure inert or inactive gas atmosphere at 30 kPa or less. Thus it is possible to directly obtain, in the condition as cast, permanent magnets that exhibit magnetic properties of iHcxe2x89xa72.5 kOe and Brxe2x89xa79 kG, with a thickness of 70 to 500 xcexcm, made up of fine crystallite having a mean crystalline grain diameter of 50 nm or less, 90% or more whereof is comprised by a crystalline structure wherein an Fe3B compound and xcex1-Fe coexist with a compound phase having an Nd2Fe14B crystalline structure.
The present invention, moreover, is a manufacturing method wherewith can be obtained thin-plate permanent magnets exhibiting magnetic properties of iHcxe2x89xa72.5 kOe and Brxe2x89xa79 kG, wherein the alloy melt noted above is continuously cast on a rotating cooling roller or rollers to yield a cast alloy having a thickness of 70 to 500 xcexcm, made up of fine crystallites having a mean crystalline grain diameter of 10 nm or less, whereafter this is subjected to a heat treatment to grow crystal grains in a temperature region of 550xc2x0 C. to 750xc2x0 C., thereby yielding a fine crystalline alloy exhibiting a mean crystalline grain diameter of 10 to 50 nm.